Top 10 Most Polluted US Cities: A Shocking Smog Crisis

When you think of the United States, perhaps things like baseball, McDonald’s, the New York Times Square, or the Grand Canyon are the first things to pop into your mind. But what about pollution? Probably not.

According to the “State of the Air” 2023 report, about 36% of Americans live in the most polluted cities in the US. The country has made strides to reduce unhealthy levels of ozone or particle pollution by cleaning up sources of air pollution, but there’s still much to be done.

So before you relocate or visit a city in the US, make sure you check the air quality. The same goes for global travel. Have a look at the most polluted cities in the world so you won’t have to deal with thick clouds of smog, trash-filled streets, or overflowed sewages on vacation.

Los AngelesPhoto byPeter ThomasonUnsplash

How Are These Polluted Cities Ranked?

Before I dive into this list of the most polluted cities in the United States, let’s unpack how the pollution levels of these cities are decided. The Air Quality Index (AQI) is an indicator used to determine how clean or polluted the air is in places around the world.

There are several aspects to consider when figuring out AQI. Factors such as particle pollution, ground-level ozone, carbon dioxide, nitrogen dioxide, and sulphur dioxide levels all affect the AQI of a city. This is measured from 0 to 400+, with 0 being good and 400+ being hazardous.

This guide unpacks cities with the highest amount of ozone, or “smog”, as per the American Lung Association’s recent State of the Air report. Ozone aggressively attacks lung tissue and causes many health issues.

If you don’t live in any of the cities mentioned, you can check the air quality of where you are on the AQI website.

1. Los Angeles-Long Beach, CA

Topping the list is the metropolitan area of Los Angeles and Long Beach. The region has again ranked number one for the most ozone-polluted area in the United States for 23 out of 24 years of the annual “State of the Air” report.

The Los Angeles-Long Beach area has a high population density, which causes a lot of competition for basic resources like food, water, and shelter. This population density translates to high vehicle emissions and an increased level of smog.

High levels of ground-level ozone pose significant health risks to the people living in this area. The chemical has been known to cause shortness of breath and asthma, ultimately reducing people’s lifespans.

2. Visalia, CA

Average number of high ozone days: 92.2

Known as the gateway to the lush Sequoia National Forest, Visalia is famous for its agricultural sector and plenty of outdoor activity opportunities. Ironically, the city is also known for having a high level of ozone pollution.

Carbon dioxide emissions and intensive farming practices mainly cause Visalia’s pollution. The city’s surrounding mountain ranges are also to blame, as their bowl-shaped landscape traps the smog within the valley.

The city has a low population density, with about 500,000 residents. However, heavy traffic in and out of the San Joaquin Valley to places like Los Angeles or San Francisco is a major factor in air pollution.

Affordable housing is in short supply in Visalia, which can lead to higher rates of homelessness. This could negatively impact the pollution levels in the city as well.

3. Bakersfield, CA

Average number of high ozone days: 86.7

Nicknamed the “breadbasket of California”, Bakersfield is famous for its fertile soil and ideal climate for growing agricultural produce. The city’s crop cultivation is expansive, ranging from fruits and nuts to cotton. However, farming chemicals are a major contributor to high levels of ozone.

Other pollutants in Bakersfield include fumes from trucks and trains and residue from the city’s oil and gas industry. Although it ranks third for ozone pollution, Bakersfield actually tops the list for the worst annual particle pollution in the United States.

Similar to Visalia, the city also has mountains around it that trap toxic chemicals from leaving the valley.
Bakersfield’s population density is relatively high with a nearly a million residents. The competition for resources is also high, so an increased crime rate and cost of living could worsen pollution in Bakersfield.

4. Fresno-Madera-Hanford, CA

Average number of high ozone days: 54

The Fresno metropolitan area lies at the heart of California’s agricultural region, but the city is more than just dust and farms. Fresno offers “big city” amenities like gorgeous recreational parks, shopping opportunities, and an amazing restaurant scene.

The city boasts an affordable cost of living, but you’ll have to deal with ozone pollution if you relocate to Fresno. The main pollutants in the city include emissions from vehicles, factories, and pesticides from farming. Smoke from wildfires in the nearby mountains is also a big contributor.

Other factors like low education levels and poverty add to the competition for resources, which leads to more combustible fuels being burned for cooking and heating. Fresno also has issues with high rates of crime, homelessness, and gun violence.

5. Phoenix-Mesa, AZ

Average number of high ozone days: 41.2

Located in the East Valley of the Phoenix metropolitan area, Mesa is a city of giant flat-topped hills, desert shrubs, and remnants of the native Hohokam people.

Arizona’s third-largest city offers plenty of outdoor excursions, as well as museums and spacious parks to explore. And let me not forget its inaugural Arizonan pumpkin patch picking opportunities throughout October.

However, Mesa ranks highly in ozone pollution. This can be attributed to common causes like vehicle and factory emissions and road repairs, which release toxic metals such as lead and mercury into the atmosphere. The intense heat in Mesa worsens the pollution, as it creates an environment for hydrocarbons to react.

The Phoenix-Mesa area has a population of almost five million residents. High population density, cost of living, and water shortages caused by the ongoing drought also affect the pollution levels.

6. Denver-Aurora, CO

Average number of high ozone days: 35.2

Considered one of the best places to raise a family, Aurora acts as a gateway to the Rocky Mountains. The city is a great place for outdoor activities like camping, hiking, and skiing, so it’s not a surprise that you may want to visit or relocate there.

Aurora has a high ozone pollution due to commuting residents’ large-scale use of vehicles. The city’s proximity to Denver makes it a hotspot for those escaping high rental prices in the state’s capital. Other pollutants, like emissions from power plants, factories, and construction sites, also affect these levels.

The Denver-Aurora metropolitan area has a population of about 3.6 million people, with a fairly moderate population density. But this city has an affordable housing crisis, which increases homelessness, negatively affecting pollution levels. A growing population has also been noted as a possible contributor, as more people means more burning of fuels.

7. Sacramento-Roseville, CA

Average number of high ozone days: 27

Roseville ranks as one of the best places to live in California. The city offers residents a little bit of everything, with great public schools, recreational hubs, and an abundance of shopping opportunities. But there’s more to Roseville than what meets the eye, especially in the air.

Heavy traffic sees tons of toxic fumes being released into the air, while raging wildfires, factory farms, and extreme heat further contribute to pollution. Roseville experiences high temperatures throughout the year, so the heat can react with pollutants from vehicle emissions on a daily basis. The city’s location in the Sacramento Valley also makes it prone to trapped air pollutants.

With a population of over 2.6 million residents, the Sacramento-Roseville metropolitan area has a relatively low population density. That may soon change as the city is undergoing a building boom. These construction sites and the increased number of vehicles will add to the pollution.

You might want to read my articles on the most polluted countries in the world and the most polluted cities in Europe.

8. San Diego-Chula Vista-Carlsbad, CA

Average number of high ozone days: 24.7

Famously known as the “Lemon Capital” for its abundance of citrus trees, Chula Vista-Carlsbad offers exciting outdoor activities along its shores, five-star resorts, and golf courses. The city’s small-town charm attracts most of its visitors, but it’s mostly the residents who have to deal with Chula Vista-Carlsbad’s air pollution.

Carbon dioxide emissions from cars and locomotives are the main driver of pollution in the city. The area’s coastal location also contributes to pollution levels through a process called “marine inversion”. This is when cooler air near the surface gets trapped by warmer air from above and ultimately traps pollutants from leaving the atmosphere.

Other factors like the burning of fossil fuels and cooking with wood, charcoal, or dung are major contributors to both indoor and outdoor air pollution. The metropolitan area has a relatively high cost of living, with residents spending more than 40% of their income on rent and utilities. This could lead to a higher level of homelessness in the city and impact the air quality.

All-in-all, this is still a lovely place to visit, for a time at least. And if the San Diego sunset spots are anything to go by, you’ll have a mesmerising time nonetheless.

9. Houston-The Woodlands, TX

Located about 30 minutes from Houston, The Woodlands is one of Texas’s best places to live. This district boasts tons of sophisticated public parks, hiking trails, and shopping opportunities. It’s also home to some of the most famous landmarks in Texas.

But “Houston, we have a problem”, and that’s pollution. With two of the biggest refineries in the United States located in this area, toxic pollutants are released into the air daily. The 400 chemical manufacturing plants in the city also add to the emissions.

The Woodlands is rapidly growing and fast on its way to becoming a city. This has caused an influx of people, but it’s not only people who are relocating. Several companies are also moving their headquarters there, which will negatively affect the air quality of the district.

10. Salt Lake City-Provo-Orem, UT

Average number of high ozone days: 23

Known for its deep-rooted Mormon heritage, powdery ski slopes, and thriving arts and film scene, the Salt Lake City-Provo-Orem metropolitan area is one of the safest cities in the US. The city boasts one of the lowest property and violent crime rates in the state and the country.

But the safety of this metropolitan area doesn’t necessarily translate into breathable air. Vehicle emissions are responsible for about 50% of the air pollution in Salt Lake County, followed by winter inversions and fumes from wood burning in homes.

The Salt Lake City-Provo-Orem metropolitan area is among the top-performing cities in the US and has experienced an urban growth boom in the last decade. This has largely impacted the city’s housing market and cost of living, which worsens air pollution.